The ability to obtain and indicate, in a useful manner, highly accurate information regarding the position of a movable member is very important in controlling tools, equipment and apparatus, monitoring processes, determining liquid level, and for many other purposes. Typically, due to the nature of the tools, equipment, apparatus or process, the environment in which they operate and the need for continuous position information, the determination of position is usually accomplished by a position sensor installed proximately to the member whose position is being sensed but out of contact therewith in order to avoid interfering with or influencing in any way the movement of the member. Moreover, the position sensed will, most usually, be transmitted to a location remote from the movable member where it may be observed, recorded or used to control the operation of equipment or processes.
One well known way of detecting the position of a movable member without actually establishing physical contact therewith is by the use of magnetic means mounted, directly or indirectly, on the movable member for magnetically interacting with other means to produce a signal indicative of the position of the movable member. For example, in U.S. Pat. No. 4,071,818--Krisst there is disclosed a method and apparatus in which a magnetic field generator is mounted on the movable member in order to be movable therewith and an elongated ferromagnetic element is positioned adjacent and parallel to the path of movement of the member. The generated magnetic field produces a change in the Young's modulus of elasticity in an adjacent region of the ferromagnetic element such that sonic strain pulses launched along the ferromagnetic element will be partly reflected from this region of Young's modulus discontinuity. The time required for a sonic strain pulse to travel from a given point on the ferromagnetic element to the region of Young's modulus discontinuity, be partially reflected therefrom and return to a known detection point provides a measure of the position of the field generator along the ferromagnetic element. Inasmuch as the positional relationship between the field generator and any point of interest on the movable member is fixed and known, when the position of the field generator is known the corresponding position of any point of interest on the movable member is also known. The shortcoming of the Krisst apparatus is that it requires the measurement of very short times. When the length of path travelled by the sonic strain pulse is relatively long, the apparatus may be acceptably accurate for certain purposes. However, as the length of path travelled becomes short, the accuracy of the Krisst apparatus decreases rapidly Moreover, the response speed of the Krisst apparatus is relatively slow since this apparatus, like all such apparatus utilizing sonic pulses, is limited by the relatively slow travel of reflected sonic pulses and the rate of decay of echoes Moreover, the accuracy and reliability of the Krisst apparatus is adversely affected by the fact that the Young's modulus of the element varies with stress and magnetization. Therefore, it varies over the length of the member at least because the stress is greater along the upper portions of the element than along its lower portions. In addition, it may vary due to differing magnetization along its length as a result of treatments to which those portions of the member may have been subjected at various stages during its manufacture.
In U.S. Pat. No. 4,194,397--Yasuda there is shown a liquid level indicator in which a bypass flow column is affixed to the side of a tank and a magnet-containing float is disposed within the column. A housing affixed to the column but remote from the tank contains a plurality of vertically spaced-apart magnet rotors which have an initial magnetic orientation in a first position wherein the south pole is on the bottom and the north pole is on the top. As the liquid level in the column rises, the north pole on the float causes the rotors to rotate about their horizontal axes to a second position wherein the north pole is on the bottom and the south pole is on the top. Conveniently, the rotors are painted in such a manner that different colors are displayed when the rotors are in the first and second positions. In this way, the magnetic orientation within the housing changes as the float moves up and down, and such changes are visually apparent from the displayed color change The Yasuda method and apparatus is of limited usefulness and applicability. A visual indication of position by color change is impractical for a wide variety of potential sensor uses and the resolution and accuracy of such an indication is severely limited. Moreover, the Yasuda apparatus does not permit the remote indication of position or the ready application of the position information to the control of tools, equipment or processes.
It is, therefore, apparent that despite the many advances in the use of magnetic means in the detection and indication of the position of a movable member, there still exists a need for a magnetic position sensor which is significantly more economical than previous sensors, which is extremely accurate and reliable and which is readily adaptable to sensing the position of a member, irrespective of its configuration or the environment in which it operates.